The Highway code states
Here that :
Highway Code wrote:
174
Box junctions. These have criss-cross yellow lines painted on the road (see 'Road markings'). You MUST NOT enter the box until your exit road or lane is clear. However, you may enter the box and wait when you want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right.
and ...
Junctions controlled by traffic lights
175
You MUST stop behind the white ‘Stop’ line across your side of the road unless the light is green. If the amber light appears you may go on only if you have already crossed the stop line or are so close to it that to stop might cause a collision.
[Laws RTA 1988 sect 36 & TSRGD regs 10 & 36]
The bit I wanted to most point out was that once you have passed the stop line on green, you can 'go' once the road becomes clear.
I always avoid stopping part way across the ped Xing. So in practice a pause before entering behind another car turning right, to ensure there is enough space then go ahead, if there is not enough space you should not enter until there is. Stopping therefore before the Ped Xing, and this might mean that you may be very close, at, or on, the stop line, or just after it, so if the exit becomes clear you can then proceed.